Obesity rates are increasing in children of preschool age, and decreased physical activity is a likely contributor to this troubling trend. Millions of American children spend several hours per day in formal preschool settings, but little is known about the physical activity levels of children in preschools. The proposed investigation is designed to describe the physical activity behaviors of children in preschools and to identify the physical and social environmental factors as well as the institutional resources, policies and practices that influence physical activity in children attending preschools. The research will be conducted in 24 licensed preschools, and subjects will be 720 children in the 3-5 year age range (30 children per school). Physical activity will be measured in each child using two objective methods: accelerometry (motion sensors) and direct observation. Each child will wear a uniaxial accelerometer during his/her hours of preschool attendance (at least 4 hours) on five days, and raw accelerometer data will be used to calculate minutes of inactivity and minutes of light, moderate, and vigorous physical activity. Also, an observational system will be used to measure the child's physical activity and hypothesized social and environmental influences on physical activity. Each child will be observed for a total of 6 hours (twelve 30-minute periods). Institutional resources, policies, and practices will be measured by structured interviews with school personnel, by direct measures of school resources (e.g., playground dimensions), and by administration of the Early Childhood Education Rating Scale (ECERS-R). A cross-sectional, observational study design will be used. To describe associations between moderate to vigorous physical activity (MVPA) and the factors that are hypothesized to influence MVPA a series of mixed model analyses will be performed. These analyses will include preschool as a random effect to account for the intact social grouping of children within preschools. Also, multivariate analyses including both individual and school level independent variables will be performed. The proposed study will address a critical public health problem, the increasing prevalence of obesity in children, by laying the groundwork for future intervention studies and for development of effective public policies.